In general, the present invention relates to an apparatus and method of shimming a magnet. In particular, the present invention is a method of determining the placement of shim elements, and an apparatus used to place the shim elements in specific locations on the pole surface of a magnet in order to shape the magnetic field to achieve greater uniformity.
Magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) is one of the most versatile and fastest growing modalities in medical imaging. As part of the MRI process, the subject patient is placed in an external magnetic field. This field is created by a magnet assembly, which may be closed or open. Open magnet assemblies have two spaced-apart magnet poles separated by a gap, and a working magnetic field volume located within the gap.
The diagnostic quality of images produced by MRI is directly related to several system performance characteristics. One very important consideration is the uniformity, or homogeneity, of the main magnetic field. In order to produce high-resolution images, the magnetic field produced in the MRI scanner must be maintained to a very high degree of uniformity. In particular, the homogeneity of the applied, constant background magnetic field directly affects the decay rate of the signal. That is, a field having a greater homogeneity has a slower decay rate. A slower decay rate results in more signal availability, allowing a greater signal-to-noise ratio, and thus better images. However, an MRI magnet initially produces a field that is usually less uniform than that required to image successfully, preferably one part per million deviation from homogeneous. At some point after manufacture, the magnet assembly must be adjusted to produce a more uniform field.
A process known as shimming is used to improve the homogeneity of the magnetic field to the necessary levels by making small mechanical and/or electrical adjustments to the overall field. Mechanical adjustments are called passive shimming, while electrical adjustments are known as active shimming. Electrical adjustments are effective because electrical current passing through a wire will produce a magnetic field around that wire. When these wires are formed into coils, the strength, direction, and shape of the magnetic field produced can be controlled by adjusting the physical and electrical parameters of the coils. Placing these coils in strategic locations as secondary magnetic field sources has the effect of adding to or subtracting from the main magnetic field in localized regions as well as over the entire pole surface, affecting the overall homogeneity of the main field. While the use of these xe2x80x9cshim coilsxe2x80x9d has allowed the homogeneity of the main MRI magnetic field to be greatly improved, there are numerous drawbacks associated with their use.
For example, the electric current in the shim coils may be unstable, resulting in an overall instability in the main magnetic field. This instability may cause xe2x80x9cghostingxe2x80x9d in the MR images. Ghosting is an interference phenomenon that appears at periodic intervals along the phase axis. These errors are unacceptable to any radiologist, who may confuse the correct position of the patient""s anatomic elements, possibly resulting in an incorrect diagnosis.
Further, shim coils are temperature sensitive. Variations in the temperature of the individual coils can cause instabilities in the main magnetic field, resulting in image artifacts. In addition, the currents used to produce the magnetic fields in the shim coils require complicated electronic circuits, such as voltage and current regulators and current amplifiers, to maintain stability. The shim coil can become inoperable when one or more of these electronic components breaks or goes out of tolerance. Even when all the electronic components are working properly, this type of active shimming adds expense and complexity to the overall MRI system. Passive shimming avoids adding complexity and expense to the MRI system, but instead requires complex calculations and a time-consuming, iterative process of modifying an arrangement of magnetic elements to shape the field.
It is common practice to represent the field in terms of a complete set of functions such as spherical harmonics, and to apply shims that attempt to reduce the amplitude of each of the functions. After one of the terms has been reduced to an acceptable level, the next term is considered. The problem with this method is that there may be many significant functions in an accurate representation of the field. To shim them one at a time is a long and tedious process.
Further, it is usually impossible to produce a shim that affects only a single one of the functions. Often the correction of one function will lead to the appearance of one of the other functions.
There is therefore a great need for a method of shimming a magnet to control the homogeneity of the resulting field that is accurate and efficient, in terms of both the time and the computational resources required to produce the desired homogeneity. The need also exists for shim elements to be used in implementing the method that provide a predictable effect on the background field, and which are simple to apply to the magnet pole.
It is therefore an objective of the present invention to provide a shimming process that determines the optimum placement location of shim elements for increasing or otherwise adjusting the uniformity of a magnetic field.
It is a further objective of the present invention to provide a shimming process that determines the optimum size of shim elements to be disposed for increasing or otherwise adjusting the uniformity of a magnetic field.
It is also an objective of the present invention to provide shimming elements that can be used in a shimming method, placement of which have predictable effects on the magnetic field.
It is another objective of the present invention to provide a holding apparatus that allows for simple placement of the shimming elements when implementing the shimming method.
To overcome the foregoing disadvantages of the active (coil) shimming process, the present invention eliminates or minimizes the use of some or all shim coils and their associated currents altogether, achieving a high degree of field uniformity required for high resolution imaging through a process using only passive shimming. The shimming is effected through the use of magnetic shim elements that are added to the standard magnet in order to physically influence the overall field produced by the magnet. The shim elements may be held in place by a non-metallic, non-magnetic position plate that is affixed to the magnet pole. The shim elements take the form of magnetic spheres. To overcome the disadvantages of conventional passive shimming techniques, the present invention also provides a process for determining the optimum sizes and locations of the shimming spheres in order to maximize field homogeneity.
According to a first aspect of the present invention, an apparatus for changing the homogeneity of a magnetic field produced by a magnet having a pole includes a shim element, and a non-magnetic plate for attachment to the magnet pole. The shim element is to be placed within the magnetic field so as to become magnetized and have an effect on the magnetic field. The plate is adapted to receive the shim element such that the shim element is disposed at a fixed position with respect to the non-magnetic plate. The shim element includes a magnetic material that is formed substantially in the shape of a sphere. The shim element may be a plurality of shim elements. The magnetic material may be a soft-ferromagnetic material. The sphere shape of each of the plurality of shim elements has a radius, and the radius of the sphere shape of one of the plurality of shim elements may be different than the radius of the sphere shape of another of the plurality of shim elements. The apparatus may also include a shim holder that is adapted to hold the shim element, wherein the plate is adapted to receive the shim holder at the fixed position. The shim holder may be a non-magnetic sphere having a bore for receiving the shim element. In this case, the plate includes a notched hole at the fixed position, such that a diameter of the hole is smaller than a diameter of the non-magnetic sphere, and a diameter of the notch is at least as large as the diameter of the non-magnetic sphere. Alternatively, the shim holder may be a non-magnetic screw having outer threads, and a bore for receiving the shim element. In this case, the plate includes a threaded hole at the fixed position for receiving the screw, such that the threaded hole mates with the outer threads of the screw. The plate may be adapted to receive the shim holder at a plurality of positions, wherein any of the plurality of positions can be selected to be the fixed position. The apparatus may also include at least one second non-magnetic plate for attachment to the magnet pole outside a peripheral area of the first non-magnetic plate. The second non-magnetic plate is adapted to receive a shim element such that the shim element is disposed in a fixed position with respect to the second non-magnetic plate.
According to another aspect of the present invention, an apparatus for changing an homogeneity of a magnetic field produced by a magnet having a pole includes a plurality of shim elements, and a non-magnetic plate for attachment to the magnet pole. The shim elements are to be placed within the magnetic field so as to become magnetized and have an effect on the magnetic field. The plate is adapted to receive the plurality of shim elements such that the shim elements are disposed at respective fixed positions with respect to the non-magnetic plate. Each of the plurality of shim elements includes a magnetic material that is formed substantially in the shape of a sphere. The magnetic material may be a soft-ferromagnetic material. The sphere shape of each of the plurality of shim elements has a radius, and the radius of the sphere shape of at least one of the plurality of shim elements may be different than the radius of the sphere shape of at least another of the plurality of shim elements. The sphere shape of each of the plurality of shim elements may have a radius that is selected from among a number of fixed radii in a series of shim element sphere radii. The apparatus may also include a plurality of shim holders that are each adapted to hold a respective one of the plurality of shim elements, in which case the plate is adapted to receive the plurality of shim holders at the respective fixed positions. Each of the plurality of shim holders may be a non-magnetic sphere having a bore for receiving the respective one of the plurality of shim elements. In this case, the plate includes a notched hole at each of the respective fixed positions, such that a diameter of the hole is smaller than a diameter of the non-magnetic sphere, and a diameter of the notch is at least as large as the diameter of the non-magnetic sphere. Alternatively, each of the plurality of shim holders may be a non-magnetic screw having outer threads, and a bore for receiving the respective one of the plurality of shim elements. In this case, the plate includes a threaded hole at each of the respective fixed positions for receiving the screw, such that the threaded hole mates with the outer threads of the screw. The plate may be adapted to receive the plurality of shim holders at a plurality of positions, wherein the number of the plurality of positions is greater than the number of the plurality of shim holders. In this case, any of the plurality of positions can be selected to be one of the respective fixed positions. The apparatus may include at least one second non-magnetic plate for attachment to the magnet pole outside a peripheral area of the first non-magnetic plate. The second non-magnetic plate may be adapted to receive a shim element such that the shim element is disposed in a fixed position with respect to the second non-magnetic plate.
According to another aspect of the invention, a process for adjusting an homogeneity of a magnetic field produced by a magnet having a pole begins by mapping the magnetic field to provide original field map values. Alternatively, at least one spherically-shaped magnetic shim element may be placed within the magnetic field so as to become magnetized and have an effect on the magnetic field prior to providing the original field map values. A sphere action is selected that will adjust the homogeneity of the magnetic field. The selected sphere action may be the adding, removing, or replacing of a spherically-shaped magnetic shim element within the magnetic field so as to become magnetized and have an effect on the magnetic field. An effect on the magnetic field caused by the selected sphere action is calculated, and the calculated effect on the magnetic field of the selected sphere action is combined with the original field map, to provide adjusted field map values. The adjusted field map values are compared with the original field map values. Based on the comparison, it is determined whether the adjusted field map values indicate a satisfactory adjustment of the homogeneity of the magnetic field. If it is determined that the adjusted field map values indicate a satisfactory adjustment of the homogeneity of the magnetic field, the selected sphere action is performed to adjust the homogeneity of the magnetic field. According to a further aspect of this process, it is determined whether the magnetic field, including the effect on the magnetic field of the selected sphere action, has an acceptable level of homogeneity, based on the adjusted field map values. If it is determined that the level of homogeneity of the magnetic field, including the effect on the magnetic field of the selected sphere action, is not an acceptable level, a second sphere action that will adjust the homogeneity of the magnetic field is selected. Also according to a further aspect of this process, it is determined, based on the adjusted field map values, if the magnetic field, including the effect on the magnetic field of the selected sphere action, has an acceptable level of homogeneity. If it is determined that the level of homogeneity of the magnetic field, including the effect on the magnetic field of the selected sphere action, is not an acceptable level, a plurality of series sphere actions are selected that will adjust the homogeneity of the magnetic field. The series is continued until it is determined that the level of homogeneity of the magnetic field, including the effect on the magnetic field of the selected sphere actions, is an acceptable level. The selected plurality of sphere actions are then performed to adjust the homogeneity of the magnetic field. According to this further aspect of this process, an effect on the magnetic field caused by each of the selected sphere actions is calculated. The calculated effect on the magnetic field of each of the selected sphere actions, is combined with the original field map to provide respective adjusted field map values. Each adjusted field map value in the series is compared with the previous field map value in the series. Based on the comparison, it is determined if the adjusted field map values indicate a satisfactory adjustment of the homogeneity of the magnetic field. If it is determined that the adjusted field map values indicate a satisfactory adjustment of the homogeneity of the magnetic field, the selected sphere action is performed to adjust the homogeneity of the magnetic field. The satisfactory adjustment of the homogeneity of the magnetic field for this process may be an adjustment that increases the homogeneity of the magnetic field.
According to another aspect of the present invention, a process for adjusting an homogeneity of a magnetic field produced by a magnet having a pole includes placing a spherically-shaped magnetic shim element within the magnetic field so as to become magnetized and have a localized effect on the magnetic field.